Nitrobenzene.1

Reaction :

The chief condition to be observed in the manufacture of nitrobenzene is the correct and intimate mixture of the components; if this is done it is easy to obtain good yields. 100 Gms. of benzene are treated with a mixture of no gms. nitric acid (sp. gr. 1.44=44° Be.) and 170 gms. concentrated sulphuric acid of 66° Be. with vigorous stirring, in a porcelain beaker provided with a well-fitting lid, or in a glass bolthead (Fig. 9). In order to ensure smooth nitration an acid of sp. gr. 1.46 (=80 % Hno3=46° Be.) may be used, but in practice an acid of 1.44 (=75 % Hno3) is quite sufficient. The internal temperature is maintained at 500 by external cooling, the addition of the acid occupying about half an hour. When all has been added the mixture is stirred for a further 2 hours at 50-600. The nitrobenzene floats on the surface of the acid, which has a specific gravity of about 1.236. The product is separated in a separating funnel, washed with a little water, then with dilute soda solution, and, finally, again with water. It is then tested with litmus and distilled. At first a little water and some benzene come over, and then pure nitrobenzene. If the benzene used is pure, excellent yields should be obtained even in the laboratory, 100 gms. of benzene, for example, yielding 150 gms. of pure nitrobenzene. B.p. 2050.

Notes on Works Technique and Practice. - Nitrobenzene is one of the most important products used in colour technology. It serves for the preparation of aniline and benzidine, and also for the production of the important Nigrosines. On the works scale for the production of nitrobenzene, charges up to 1500 kgs. of benzene are employed, yields of 98 % being obtained. With such large charges the operation lasts about 12 hours, about 97 % of the nitric acid being used up. The course of the reaction is usually followed by a quantitative determination of the amount of nitric acid remaining in the acid mixture. The waste acid should not contain finally more than 1 % of nitric acid (estimation in Lunge nitrometer). The nitrobenzene is usually used without further purification, but to obtain it in a pure condition it is always distilled in vacuo.

Figs. 10 and 11 (Plate IV.) show a nitrating vessel with internal

100 gms. Benzene, no gms. Hno3, sp.gr. 1.44. 170 gms. H2So4, 66° Be.

1 Cf. also Ullmann, Enzyklopadie d. Techn. Wissenschaften, cooling as used for aromatic hydrocarbons, and also a separating funnel with a sight glass (or so-called "lunette") and a lead or stoneware tap affixed beneath. The nitrating plant for benzene must be homogeneously lead-lined as the waste acid obtained at the end attacks iron owing to its too great dilution.

m-Dinitrobenzene from Nitrobenzene.

Reaction:

1:3-Dinitrobenzene, usually referred to simply as dinitrobenzene, is obtained from mono-nitrobenzene. For this purpose the crude nitrobenzene is always employed. It is only necessary to run off the waste acid after the first mono-nitration has been effected. It is not possible to treat benzene straight away with excess of nitric acid as explosions may occur. It is also absolutely essential that the stirring be as vigorous as possible, as insufficient mixing may be extremely dangerous, particularly on the large scale. If, owing to the stopping of the stirrer, two layers should form, consisting of hydrocarbon on the one hand and nitrating acid on the other, the acid should at once be run off with the stirrer stationary. Cases have been known in the industry where terrible explosions have occurred on subsequently restarting the stirrer owing to sudden overheating (e.g. Rummelsburg, near Berlin).

123 Gms. nitrobenzene are placed in a sulphonating pot of 500 c.cs. capacity. A mixture of 450 gms. concentrated sulphuric acid of 66° Be. and 140 gms. nitric acid of 470 Be. (sp. gr. 1.48=88 %) is allowed to drop in at 100° during half an hour, with very efficient stirring, which is best effected by means of a propeller stirrer or a Witt's bell-stirrer which must dip right into the liquid. The temperature may be allowed to rise to 1150, and the addition of the acid is so regulated that this temperature is not exceeded. After all has been added, the stirring is preferably continued for a further half-hour. The sulphonating pot is covered with a divided sheet of lead so as to prevent the escape of vapours. The nitration is practically quantitative.

The mixture is then allowed to cool to about 700, and is poured into half a litre of cold water with good stirring. Some nitrous fumes are evolved (fume cupboard), and the crude dinitrobenzene is at once precipitated as a solid, crumbly mass. The waste acid is decanted off and the residue melted up with about half a litre of water. After cooling and pouring off the washing water, the operation is repeated with the addition of sufficient soda to render the solution strongly alkaline to litmus. Finally, the dinitro product is swirled round at 8o° with 500 c.cs. of water, to which 10 c.cs. of 30 % soda lye have been added. A dinitro product is obtained in this way which has a solidifying point of about 8o°, and remains practically uncoloured with soda solution; it is dried in a drying chest at 900, and, on cooling, a crystalline cake is obtained weighing about 150 gms. Note on Works Practice. - The technical product is not usually quite so pure as it contains about 3 % p-dinitro- and 1 % o-dinitro-benzene (see also under m-Phenylenediamine). Dinitrobenzene is an extremely poisonous substance and quite as dangerous as prussic acid. The workmen who deal with it must always change their clothes in the works and wear gas masks. The substance can even penetrate through the skin into the blood and causes acute Cyanosis, a form of poisoning in which the lips of the patient become blue, the pulse weakens, and frequently death supervenes after long illness.

123 gms. Nitrobenzene.

450 gms. H2S04, 66° Be. 140 gms. Hno3, 470 Be.

10 c.cs. NaOH

(30 %).